When assembling a woodworking project, it's often important to know if something is "square" before it assembled, or when performing the final assembly. The easiest way to check for squareness doesn't require any type of square device -- all you need is a tap measure.

Using the tape measure, measure from one corner to the corner on the other side of the diagonal. Then measure the other diagonal. If the distances are the same, then you are square. If they are different you are not square. Pretty simple.